There is a need for improved polymeric materials that are highly permeable, yet may under certain circumstances, provide selective separation of various gas combinations. Such materials would especially be useful in commercial, non-cryogenic gas separation processes.
The commercial application for gas separation devices based on polymeric materials relies, in part, on maximizing the overall gas flux through the membrane. P. H. Kim, et al., J. Appl. Poly. Sci., 34 1761 (1987), reported that the gas flux for a membrane is relatable to the average space between the polymer chains. In addition, they indicated that the density of the polymer is also related to the overall gas flux. The problem, in part, for these commercial applications is to identify polymers with very high flux and with good thermo-mechanical properties. It has generally been observed that to achieve high overall flux requires having a polymer with low chain-chain interactions. This can be exemplified by polymers such as poly(dimethylsiloxane) or poly(4-methyl-1-pentene). These materials have rather high gas flux values. These high flux materials have, because of their low chain-chain interaction, low glass transition temperatures (Tg). As a consequence, these materials require either special processing conditions to build in chemical and physiochemical crosslinking or they can be used only at rather low application temperatures. By contrast, polymers with strong chain-chain interactions have rather high Tg values and have usually exhibited rather low gas flux. U.S. Pat. Nos. 3,822,202 and 3,899,309; U.S. Pat. No. Re 30,351 (1980), disclose a process for separating fluids using a semi-permeable membrane made from polyimides, polyesters or polyamides. The repeating units of the main polymer chain of these membranes are distinguished in that such repeating units have at least one rigid divalent subunit, the two main chain single bonds extending from which are not colinear, is sterically unable to rotate 360.degree. around at least one of these bonds, and has 50% or more of its main chain atoms as members of aromatic rings.
U.S. Pat. No. 4,705,540 discloses a highly permeable aromatic polyimide gas separation membrane and processes for using said membrane. The membrane is an aromatic polyimide membrane in which the phenylenediamines are rigid and are substituted on essentially all of the positions ortho to the amino substituents, and the acid anhydride groups are essentially all attached to rigid aromatic moieties.
U.S. Pat. Nos. 4,717,393 and 4,717,394 disclose polymeric membranes and processes using the membranes for separating components of a gas mixture. The membranes disclosed in both of these patents are semi-flexible, aromatic polyimides, prepared by polycondensation of dianhydrides with phenylenediamines having alkyl substituents on all ortho positions to the amine functions, or with mixtures of other, non-alkylated diamines, some components have substituents on all positions ortho to the amine functions. It is taught that the membranes formed from this class of polyimides exhibit improved environmental stability and gas permeability, due to the optimization of the molecular free volume in the polymer.
U.S. Pat. No. 4,378,400 discloses gas separation membranes formed from aromatic polyimides based upon biphenyltetra-carboxylic dianhydride for separating various gas mixtures. Japanese Patent Applications 1-194904 and 1-194905 disclose gas separation membranes formed from various polyarylates and polyimides respectively.
Japanese Patent Applications 63-190607 and 63-278524 disclose gas separation membranes formed from polyamides. The polyamides disclosed as being useful for such membranes are characterized as having hydrogen at all positions ortho to the amine functions or, alternatively, as having a maximum of only one ortho group to each amine function in the alkylated form. Additionally, both applications limit the o-alkyl group to either methyl or ethyl.
M. Salame in Poly. Eng. Sci., 26 1543 (1986) developed a predictive relationship for oxygen permeability coefficient [(PO.sub.2)] and polymer structure. In the publication he demonstrates the group contributions of various structural portions of a polymer to P(O.sub.2) values. In particular he indicates the presence of an aromatic group, such as phenyl, in place of a methylene (--CH.sub.2 --) decreases the P(O.sub.2) values for a pair of comparative polymers.